If you're experiencing trouble with logging into Charity Miles via Facebook, there's one way to fix this.
Log into your account with your email account on the Log In screen. After this, tap the Main Menu button in the top left corner of the screen. Next, select Profile. Here you'll be able to sync your account with Facebook by tapping the Edit button in the top right corner. Lastly, select the Sync My Account with Facebook button at the button of the Edit Page. After doing this, you should now be able to log into Charity Miles through your Facebook account.